[libTooling] In Clang Transformer, change `Metadata` field to deferred evaluation.

`Metadata` is being changed from an `llvm::Any` to a `MatchConsumer<llvm::Any>`
so that it's evaluation can be be dependent on on `MatchResult`s passed in.

Reviewed By: ymandel, gribozavr2

Differential Revision: https://reviews.llvm.org/D83820